Would most people appreciate their lawn care person switching them to bi-weekly cutting if became hot & dry?

Sounds like you really want to give your client a good service -they are lucky. Cutting the grass too often when it is dry could actually damage it which your client is not going to appreciate. However, if they don't want to lose you they would be glad to have you come and do something else to keep their slot open. I would chat to the client about doing this - They would appreciate your honesty.